The present invention relates to solid-state laser devices and to integrating spheres.
On account of their compactness and ease of use, solid-state lasers as typified by the YAG laser have been used extensively and especially in the laser machining field. Recently, they have also found wide application in the measurement and medical fields.
A difficulty with solid-state lasers lies with raising their oscillation efficiency. Typically in solid-state laser devices, a laser medium is in the shape of a rod or a slab (plate), and an electric discharge tube, e.g., a krypton flash lamp disposed parallel to the laser medium, is used for exciting the laser medium to emit light for oscillation. For laser efficiency, attention is due the way exciting light is injected. For injection, reflectors made from various highly reflective materials have been used. Most common among these are gold-plated copper or brass reflectors. Also known are silver-plated reflectors and ceramic reflectors, for example.
A similar need for high reflectance arises with so-called integrating spheres as used in optical measurement systems. If a conventional optical detector is used for measuring light energy, for example, the measured value may depend on the size of a light-receiving surface and on the intensity distribution of incident light across the light-receiving surface. Thus, different beams with the same energy may produce different measurement readings. For more consistent readings, an integrating sphere can be used in which injected light is repeatedly and diffusely reflected at its inner surface so that the intensity distribution of the light is made uniform. For high reflectance of the inner surface of the integrating sphere, white coatings having MgO or BaSO.sub.4 as their main component are typically used.
Gold-plated reflectors as mentioned above have desirably high reflectance especially for light of wavelength near 0.8 .mu.m, which contributes greatly to the oscillation of YAG lasers, with little decrease in reflectance of the plated surface due to contamination and deterioration, for example. However, since there is considerable absorption loss for light in the 0.5 to 0.6 .mu.m absorption band of Nd:YAG crystals, with the reflectance of gold-plated surfaces falling off at wavelengths below 0.6 .mu.m, excitation efficiency remains limited. Furthermore, with laser crystals doped with Cr.sup.3+ such as Cr:Nd:GSGG and Cr:BeAl.sub.2 O.sub.2 (alexandrite), because of large absorption in the 0.5 to 0.6 .mu.m wavelength band, gold-plated reflectors are not suitable for solid-state laser devices including such a crystal as laser medium.
As reflectors at short wavelengths, silver-plated reflectors are problematic because silver forms sulfides having decreased reflectance. This difficulty can be overcome, e.g., by coating the silver surface with a protective film of SiO.sub.2, for example, or by silver-plating the back surface of a glass plate so that the silver surface does not come into direct contact with cooling water. In high-power lasers, however, silver-plate surfaces are less suitable because of the likelihood of damage caused by heat produced by the excitation light.
As reflector materials at short wavelengths, ceramic materials have also received recent attention, but the reflectance of such materials may not always be sufficient. Reflectance may be lower still with reflectors in which so-called free-cutting ceramics are used, made by dispersing ceramic particles in a glass matrix.
Similar difficulties arise with integrating spheres. Although, for weak light, the above-mentioned coatings containing MgO, BaSO.sub.4, or the like are well suited, at higher powers their resistance too light becomes unsatisfactory.